waecE-LEARNING
Chemistry Paper 2 (Essay) ,May/June 2014  
Questions:   1 2 3 4 5  Main
General Comments
Weakness/Remedies
Strength













































































Question 2

2.         (a)        Define each of the following terms:  
                                    (i)      ion;       

                                    (ii)     isotopes.                                                  [4 marks]

                        (b)        (i)      Consider the elements 12Mg and 13Al.

                                    I.       Write the electron configuration of each element.

                                    II.      Explain briefly why the first ionization energy of 12Mg
                                              is greater than that of 13Al.                                                    [8 marks]
                        (c)        (i)      What are allotropes?

                                    (ii)     Name the two crystalline allotropes of carbon.

                                    (iii)    Give one industrial use of each allotrope name
                                               in 2(c)(ii).                                                [6 marks]

                        (d)       On warming crystals of sodium chloride with concentrated
                                    tetraoxosuphate (VI) acid, a gas was evolved.
                                   
                                    (i)      List two physical properties of the gas produced.

                                    (ii)     Write a balanced equation for the reaction.                          [4 marks]
           
                        (e)        A certain chip W used in a circuit of a microcomputer has a
                                    mass of 5.68 mg.  
                                    Calculate the amount of W in the microcomputer
                                                            [ W = 28 gmol-1]                                                      [3 marks]

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OBSERVATION

This question was popular among the candidates and their performance
            was good.

            In part (a), majority of the candidates define isotopes correctly.  However,
            some candidates defined isotopy instead of isotopes while others in their
            definition of isotopes mentioned “elements” instead of “atoms or the
            same elements”.

            In part (b), majority of the candidates where able to write the electron configuration
            of Mg and Al.  However, some candidates could not correctly explain why the first
            ionization energy or Mg is greater than that of Al.

            Candidates could not satisfactorily give three different oxides of period 3 elements           that react with water.

            In part (c), majority of the candidates were able to define allotropes and name the             crystalline allotropes of carbon.

            In part (d), majority of the candidates could not list correct physical properties of the
gas evolved on warming crystals of NaCl with Conc. H2SO4.  Some candidates could not write a correct balanced chemical equation for the reaction although a few  did.

            In part (e), most candidates could not convert 5.68mg to gram and therefore could            not correctly calculate the amount of W in the microcomputer.  However, a few did.

            The expected answers include:

            (a)        (i)       ion         -           electrically charged atom or group of
atoms formed from an atom by loss or gain of electron.

                        (ii)     isotopes  -           atoms of the same element with the same
number of protons but different number of
            neutrons / mass number.     

                       
          (b)  (i)    I.          12Mg                -           1s22s22p63s2                
                                    13Al                  -           1s22s22p63s22pl           

                        II.        The 3p electron in Aluminium is slightly further away
                                    from the nucleus than the 3s electron of Magnesium. 
                                    The shielding/screening effect by the inner electrons is
                                    more in Al than in Mg.  These two effects outweigh
the increase in nuclear charge of Al, hence nuclear
attraction for 3s is greater than the 3p
                                                                        OR
The 3s outermost orbital in Mg is fully filled and is
therefore more stable than the 3p orbital in Al which
is neither completely filled nor half filled and hence
less stable
OR
3s orbital of Mg is closer to the nucleus than the 3p
orbital of Al hence the nuclear attraction for the 3s is
greater than the 3p  

                        (ii)        Na2O, MgO, P4O10, SO3, Cl2O7, SO2, Na2O2, NaO2
 

            (c)        (i)         Allotropes are different forms of the same element in the same
            physical state. 
        

 

                        (ii)        Diamond   and graphite   

                        (iii)  Diamond -           for making jewellery
-           for cutting hard surface materials
-           making abrasives
                                     
                        Graphite          -           As a lubricant
      -           for making pencils
      -           for making electrodes
      -           neutron moderator
      -           making crucibles etc
                                           
             (d)     (i)                      -           colourless
                                                -           has choking smell
                                                -           denser than air
                                                -           fumes in moist air
                                                -           soluble in water
                                                                       
                        (ii)        2NaCl + H2SO4 → Na2SO4 + 2HCl              
                                                OR
                                    NaCl + H2SO4→NaHSO4 + HCl
                                                                                                                                     
            (e)        (i)         Mass of Q = 5.68mg = 5.68 x 10-3g               
                                    Moles of Q =         

                                                       = 2.03 x 10-4 moles   
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